In a UMTS (Universal Mobile Telecommunications System) network, for the purposes of improving spectral efficiency and improving peak data rates, system features based on W-CDMA (Wideband Code Division Multiple Access) are maximized by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access). For this UMTS network, for the purposes of further improving spectral efficiency, increasing peak data rates, providing low delay and so on, long-term evolution (LTE) has been under study (see Non Patent Literature 1).
In LTE, unlike in W-CDMA, as multi access schemes, an OFDMA (Orthogonal Frequency Division Multiple Access)-based system is adopted for the downlink and an SC-FDMA (Single Carrier Frequency Division Multiple Access)-based system is adopted for the uplink.
Signals to be transmitted on uplink are, as illustrated in FIG. 1, mapped to appropriate radio resources and transmitted from a user terminal (UE (User Equipment) #1, UE #2) to a radio base station. In this case, user data is allocated to an uplink shared channel (PUSCH: Physical Uplink Shared Channel). Control information is multiplexed with the PUSCH when being transmitted simultaneously with the user data, or is allocated to an uplink control channel (PUCCH: Physical Uplink Control Channel) when the control information is only transmitted.
The control information to be transmitted on uplink includes downlink quality information (CQI: Channel Quality Indicator), transmission acknowledgement signals (ACK/NACK) in response to downlink shared channel (PDSCH: Physical Downlink Shared Channel) signals.
In a third-generation system (W-CDMA), it is possible to achieve a transmission rate of maximum approximately 2 Mbps on the downlink by using a fixed band of approximately 5 MHz. In an LTE system, it is possible to achieve a transmission rate of about maximum 300 Mbps on the downlink and about 75 Mbps on the uplink by using a variable band which ranges from 1.4 MHz to 20 MHz. In the UMTS network, successor systems to LTE are also under study for the purposes of further improving spectral efficiency and increasing peak data rates (for example, which may be called “LTE advanced” or “LTE enhancement” (hereinafter referred to as “LTE-A”)).
Further, in the LTE-A system, study has been made about HetNet (Heterogeneous Network) in which a small cell (for example, pico cell, femto cell, RRH or the like) having a local coverage of about several ten-meters radius is arranged within a macro cell having a wide coverage of several-kilometer radius (for example, non-patent literature 2). HetNet is a radio communication system in which the macro cell and the small cell are arranged at least partially in an overlapping manner.